1. Field of the Invention
This invention relates to a friction material that is suited for use in a manual or automatic transmission drive assembly. Particularly, the invention relates to a friction material comprised of a heat resistant paper support which has a friction coating formed of plural layers of carbon particle-filled thermoset resin which includes granular carbon friction particles. The invention also relates to a method of making the friction material by coating the paper with a liquid dispersion of thermosettable binder material and carbon particle filler, introducing granular carbon friction particles and curing. The invention also provides a transmission which includes the friction material.
2. Background
Transmissions are utilized to engage and disengage a drive means between a motor or engine and its driven parts. Both manual and automatic transmissions are well known. Such devices typically utilize a member which bears a friction material to effectively engage these parts to efficiently operate under conditions which permit optimum performance with a long useful life. Manual transmissions typically utilize a blocker ring which bears a friction facing on its curved surface as an engaging means. The frictional properties of the friction material for automatic transmissions should be selected to provide relatively constant frictional engagement over a wide range of slip speeds to minimize transmission shudder. Heretofore a wide variety of friction materials have been utilized in transmissions. Early friction materials have included asbestos because of its good high temperature stability but asbestos is no longer thought to be an acceptable material because of environmental and health concerns. Various friction materials have included paper substrates impregnated with a thermoset binder but such materials typically do not have good wear and/or heat resistance to the engagement loads and energies normally encountered in medium and heavy duty transmission applications and they generally cannot maintain a desirable coefficient of friction for certain automatic transmission applications. Other friction materials have included granular or fibrous carbon or other similar materials bonded together by a thermoset binder. Such materials have a high internal strength and high energy absorbing capacity, but they have poor conformability due to their rigidity and require costly additional processing when being installed.
Particularly useful friction materials must have a wide variety of acceptable properties. The friction material must be compliant but resistant to compression, fatigue, abrasion and heat resistant, have desirable and sustainable frictional properties, and a long life. If any one of these properties is not satisfied, the friction material may fail. All of these properties must be present in a friction material which is utilized in a manual transmission blocker ring. For example, the friction material must be compliant to be conformable to a blocker ring surface of a manual transmission. If conformability, is not present, the friction material can prevent reliable manufacture and/or result in a weakened structure, thereby prematurely rendering inoperative the manual transmission. Various suggestions have been made for improved friction materials, but these have been without complete success. These include:
U.S. Pat. No. 4,320,823 (Covaleski et al.) teaches an improved friction member comprising a continuous bundle or tape of aramid fiber impregnated with and bonded together by a heat curable cement comprising water soluble phenolic resin and a heat curable elastomer. The friction member may also contain friction modifiers such as carbon black, graphite, metal oxides, metal powders, barytes, clay, silica, alumina, cryolite, litharge and the like.
U.S. Pat. No. 4,374,211 (Gallagher et al.) teaches a non-asbestos material containing a thermosetting binder, a non-asbestos fibrous reinforcing material such as carbon, metal (e.g., steel) or inorganic (e.g., ceramic) fibers and up to 15% by weight based on the total weight of other ingredients of an aramid polymer pulp fiber.
U.S. Pat. No. 4,639,392 (Nels et al.) discloses a clutch plate member consisting of a friction facing layer bonded to a rigid metal plate by a layer of adhesive. The friction facing layer consists of a compliant carrier sheet such as a friction paper composed of cellulose fibers saturated with liquid phenolic resin coated by compression molding of a granular mixture of carbon and phenolic thermoset binder.
United Kingdom Patent No. 1,451,864, published Oct. 6, 1976, discloses a friction element composed of a porous industrial carbon material which may be secured to a support such as a metal plate.
West German Patent No. 1,525,334, published Sept. 30, 1971, discloses a friction material comprising carbon in an elemental form, wherein the carbon content is less than 50% by weight.
As far as is known, there is no disclosure of a friction material for use in transmissions which includes carbon particles which are bonded by plural binder layers or a support formed substantially of aramid paper.
While aramid filaments in a bundle or tape have been suggested as an integral reinforcing component of a friction material, there is no disclosure or suggestion that an aramid paper could successfully be used as a support for a friction material. On the contrary, the only reference which suggests using aramid polymer fiber limits the amount of such fiber to 15% by weight of the total weight of other ingredients. It is not surprising that papers formed entirely of aramid pulp fiber have not been used even though they are known, since such aramid papers have a very poor green strength which causes them to disintegrate when subjected to liquid coating compositions.